1. Field of the Invention
The present invention relates to a method of preparing a light emitting device equipped with a light emitting element having a layer composed of an organic compound (hereinafter, referred to as organic compound layer), and particularly relates to a multi-color display function.
2. Prior Art
Since a spontaneous emission type display device using a light emitting element in which an organic compound layer is intervened between a pair of electrodes has a wide angular field of view and is also excellent in visibility, it has drawn the attention as a display device of the next generation.
A light emitting mechanism of a light emitting element is considered as the phenomenon that a hole injected from an anode which is one of a pair of electrodes and an electron injected from a cathode which is the other one of a pair of electrodes recombine in a luminescent organic compound layer (light emitting layer) and form an exciton, and the exciton emits the light at the time when the exciton returns to the ground state. The emission is referred to as electroluminescence. The electroluminescence includes fluorescence and phosphorescence, that is, these are emission (fluorescence) from the singlet state in the excited state and emission (phosphorescence) from the triplet state. Since the brightness due to the emission ranging from several thousands to several tens of thousands cd/m2, in principle, it is considered to be capable of being applied to a display device or the like.
In such a light emitting element, an organic thin film is usually formed in a thin film less than 1 μm. Moreover, since the light emitting element is a spontaneous emission type element in which the organic thin film itself emits the light, it is not necessary to be equipped with a back light used in the conventional liquid crystal display. Therefore, it is a big advantage that a light emitting element can be easily prepared in an extremely thin type and light weight.
Moreover, a method referred to as an active matrix drive has realized a high definition image display without any crosstalk by providing a thin film transistor (TFT) per each pixel and controlling an individual emission of the light emitting element.
In a method of displaying in colors, a method in which the materials or additives of the light emitting layers are made different and luminous colors per each pixel which are made different in red, green and blue are arrayed and a method in which a light emitting element for emitting white color light is used and combined with color filters and the like have been known.
However, when the color filter and pixel matrix in which a light emitting element is provided in each pixel are individually formed, it is necessary to align both with a high accuracy and assemble them, and in the case where the alignment accuracy is low, there has been a problem that the actual aperture efficiency is lowered.
Needless to say, it is also considered that in the fabrication steps of a TFT substrate, the colored layers of the color filters are prepared with the same alignment accuracy. However, since the heat resistance temperature is about 200° C., the process temperature of about 450° C. in the TFT could not be withstood.
Moreover, a material referred to as a color resist used at the time when the colored layers of the color filters are formed is considerably expensive material, and there has been a problem that such a process in which the yield of TFT steps and color filter steps are integrated is a factor of increasing the fabricating cost and thus is not suitable.